This invention is for use with a solid-fuel-fired boiler with high combustion and system efficiency. The high level of emissions and low efficiency associated with the use of solid fuels has been an obstacle to the transition from oil to solid fuels. There is a clear need for a suitable solid-fuel-fired boiler that fulfills strict environmental standards and achieves high efficiency performance as well.
A solid fuel, e.g. wood in various forms such as logs, chips, pellets or peat, differs fundamentally from oil in its combustion properties. For example, wood burns in two widely differing phases: the Gas Combustion Phase and the Charcoal Phase. Both emissions and heat are formed and emitted in these two phases. In the former phase about 80% of the fuel mass is converted to gases in a relatively short time. Thus the gas volume and the rate of emission of the volatile matter depend on an important factor, the moisture content of the fuel. High moisture levels result in a long gas combustion phase. For a conventional boiler it has been shown that the gas combustion phase is critical from the environmental and heat transfer viewpoint. There are many physical and chemical factors at work during the gas combustion phase that affect the pattern of emissions. They will not be dealt with here. The most important factor in this context is the air supply, which will be discussed shortly.
In general, the charcoal phase involves about 20% of the total fuel mass, although the combustion time can actually be longer than that for the gas phase. The charcoal phase is not so much of concern regarding emissions, mainly because of the even and uncomplicated combustion. Even so, the supporting grate should be designed and shaped correctly to maintain a high combustion efficiency.